dt-bindings: rtc: microcrystal,rv3032: Add missing type to 'trickle-voltage-millivolt'

'trickle-voltage-millivolt' is missing a type definition. '-millivolt' is
not a standard unit (should be '-microvolt'). As the property is already
in use, add a type reference.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220719215143.1877566-1-robh@kernel.org